An ion gradient is a concentration gradient of ions, it can be called an electrochemical potential gradient of ions across membranes. Ionophores are important for ion gradients. The chemiosmotic potential is used as energy storage, chemiosmotic coupling is one of several ways a thermodynamically unfavorable reaction can be driven by a thermodynamically favorable one, I.E by letting the ions diffuse through the high concentration side to the low concentration side through transmembrane proteins which can use them as energy. Several different types of ion gradients exists in biological systems, the most common one is the proton gradient. Transmembrane ATPases or transmembrane proteins with ATPase domains are often used for making and utilizing ion gradients. The enzyme Na+/K+ ATPase use ATP to make a sodium ion gradient and a potassium ion gradient.
The proton gradient can be used as an intermediate energy storage for heat production and flagellar rotation . Additionally it is an interconvertible form of energy in active transport, electron potential generation, NADPH synthesis and ATP synthesis/hydrolysis.
Some archaea, most notably halobacteria, make proton gradients by pumping in protons from the environment with the help of the solar driven enzyme bacteriorhodopsin, here it is used for driving the molecular motor enzyme ATP synthase to make the necessary conformational changes required to synthesize ATP.
Proton gradients are also be made by bacteria by running ATP synthase in reverse, this is used to drive flagellas.
The F1FO ATP synthase is a reversible enzyme. Large enough quantities of ATP cause it to create a transmembrane proton gradient, this is used by fermenting bacteria which do not have an electron transport chain, and hydrolyze ATP to make a proton gradient, which they use for flagella and transport of nutrients into the cell.
In respiring bacteria under physiological conditions, ATP synthase generally runs in the opposite direction, creating ATP while using the protonmotive force created by the electron transport chain as a source of energy. The overall process of creating energy in this fashion is termed oxidative phosphorylation. Same process takes place in mitochondria, were ATP synthase is located in the inner mitochondrial membrane (so that F1-part sticks into mitochondrial matrix, were ATP synthesis takes place).